From: Eric Dumazet <edumazet@google.com>
To: "David S . Miller" <davem@davemloft.net>
Cc: netdev <netdev@vger.kernel.org>,
Eric Dumazet <edumazet@google.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
Jakub Kicinski <jakub.kicinski@netronome.com>,
Zhang Yu <zhangyu31@baidu.com>, Wang Li <wangli39@baidu.com>,
Li RongQing <lirongqing@baidu.com>,
Jason Wang <jasowang@redhat.com>
Subject: [PATCH net] tun: remove possible false sharing in tun_flow_update()
Date: Wed, 9 Oct 2019 09:20:02 -0700 [thread overview]
Message-ID: <20191009162002.19360-1-edumazet@google.com> (raw)
As mentioned in https://github.com/google/ktsan/wiki/READ_ONCE-and-WRITE_ONCE#it-may-improve-performance
a C compiler can legally transform
if (e->queue_index != queue_index)
e->queue_index = queue_index;
to :
e->queue_index = queue_index;
Note that the code using jiffies has no issue, since jiffies
has volatile attribute.
if (e->updated != jiffies)
e->updated = jiffies;
Fixes: 83b1bc122cab ("tun: align write-heavy flow entry members to a cache line")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Zhang Yu <zhangyu31@baidu.com>
Cc: Wang Li <wangli39@baidu.com>
Cc: Li RongQing <lirongqing@baidu.com>
Cc: Jason Wang <jasowang@redhat.com>
---
drivers/net/tun.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 812dc3a65efbb9d1ee2724e73978dbc4803ec171..a8d3141582a53caf407dc9aff61c452998de068f 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -526,8 +526,8 @@ static void tun_flow_update(struct tun_struct *tun, u32 rxhash,
e = tun_flow_find(head, rxhash);
if (likely(e)) {
/* TODO: keep queueing to old queue until it's empty? */
- if (e->queue_index != queue_index)
- e->queue_index = queue_index;
+ if (READ_ONCE(e->queue_index) != queue_index)
+ WRITE_ONCE(e->queue_index, queue_index);
if (e->updated != jiffies)
e->updated = jiffies;
sock_rps_record_flow_hash(e->rps_rxhash);
--
2.23.0.581.g78d2f28ef7-goog
next reply other threads:[~2019-10-09 16:20 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-09 16:20 Eric Dumazet [this message]
2019-10-10 3:59 ` [PATCH net] tun: remove possible false sharing in tun_flow_update() Jason Wang
2019-10-10 4:29 ` Jakub Kicinski
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